To avoid mistakenly adding multiple xlink targets to the same source when creating sync views and to make sync view creation to repos with xlinks easier I suggest that the sync view editor should be "target" centric, not "source" centric:

I click add new sync view.
Instead of clicking add new source as I am supposed to now, I click "add new target".
I choose the target on a target server.
Plastic detects that I already have a repo with the same name on my source server (or use some other similarity measure as I have suggested above) and ask if I want to use that source repo, another source repo or create a new.
Plastic detects that the target repo has xlinks. For each xlink Plastic repeats the above check for a source repo and offers to use that, another or a create a new.

By using a target first approach, this would eliminate the risk of the above error and be very convenient.
Combine that with a button for syncing all targets and you have a near perfect sync view system.